This invention relates to an inductively coupled plasma mass spectrometer (hereinafter referred to as ICP-MS) that makes it possible to perform identification and measurement of infinitesimal impurity quantities in a sample solution.
The prior art will be described with reference to FIG. 2 which shows a sample introduction portion of an ICP-MS. In FIG. 2, numeral 1 is a sample solution, numeral 2 is a capillary tube, numeral 3 is a nebulizer, or sprayer for creating a fine spray, numeral 4 is an adapter, numeral 5 is a spray chamber, numeral 6 is a drain receptacle, numeral 7 is a plasma torch, numeral 8 is a work coil, numeral 9 is a gas flow controller, numeral 10 is a high frequency power source, numeral 11 is a plasma and numeral 12 is a mass detector.
The sample solution 1 to be analyzed is introduced into the nebulizer 3 through the thin tube-shaped capillary tube 2. At the center of the nebulizer 3, there exists a thin tube which is connected to the capillary tube 2. In nebulizer 3, a gas (hereinafter called nebulizer gas) is caused to flow around the thin tube from the gas flow controller 9. When the nebulizer gas flows through the nebulizer 3, the sample solution 1 is sprayed in the form of a mist into chamber 5 via the top end of chamber 5. Nebulizer 3 has an outlet end which faces into chamber 5 and is provided with an outlet nozzle which forms the sample solution spray. This nebulizer 3 is called a coaxial type nebulizer, but so-called cross-flow type nebulizers also exist. The output end of the nebulizer 3 is connected to the spray chamber 5 by way of the adapter 4. Thus, the sample solution 1 is sprayed into the spray chamber 5. The spray chamber 5 introduces particles having diameters in a specific limited portion of this range. The mist sprayed into spray chamber 5 consists of sample solution particles having a range of diameters to the plasma torch 7 together with the nebulizer gas (this process is called classification). The other mist particles are discharged to the drain 6.
The plasma torch 7 has a triple tube structure, i.e., three tubes nested within one another. The center tube of the plasma torch 7 is connected to the spray chamber 5, and a plasma gas and an assist gas are supplied respectively to the outer tube and the middle tube from the Gas flow controller 9. The plasma gas and assist gas are usually argon.
The work coil 8 is would around the output end of plasma torch 7 so that high frequency power is supplied from high frequency power source 10. The high frequency power is usually supplied at a power level of between 0.8 and 2.0 Kw. When high frequency power is supplied to the work coil 8, and gas flows through plasma torch 7, the plasma 11 is generated and maintained because the gas is inductively coupled with an alternating magnetic field near the work coil 8. The sample solution 1 in the form of a mist introduced into the plasma torch 7 along its axis is ionized in the plasma 11. The ionized sample solution is then introduced into mass detector 12.
The mass detector 12 functions to separate the introduced ions according to mass and to detect the separated ions. Infinitesimal impurity amounts in the sample solution 1 are identified from the detected mass of ions and measured by the detected mass count of the ions.
The structure of such an ICP-MS is disclosed in, for example "The base and application of the ICP Atomic Emission Spectrometer" by Haraguchi, published by Kodansha Scientific.
In the prior art, the gas (argon) and elements of the sample solution, which constitute the plasma, are combined with each other and become molecular ions. The molecular ions are, for example, ArO ions (mass number is 56), or ArH ions (mass number is 39), etc. Therefore, the analytical performance for impurity elements, for example 56Fe, 39K, that have the same mass number as the molecular ions, is decreased a great deal by the influence of interference.